The present invention concerns an objective for an observation device, in particular for a microscope, as well as a microscope and a method for adjusting an objective.
Microscopes, in particular, stereomicroscopes, are used, for example, in order to be able to provide the surgeon with the image of the site to be treated as an enlarged image during operations. Operating microscopes are usually constructed according to the telescopic principle and essentially comprise three optical components, namely a principal objective, an afocal zooming system and a binocular telescopic tube. At the interfaces of these optical components, the beam bundles run parallel, i.e, the image is situated in infinity. Under these assumptions, these optical components can be exchanged in a very simple way for other components, as in a modular system. Thus, for example, one principal objective can replaced by another principal objective.
Such an exchange, in which, for example, an objective of one focal length is replaced by an objective of another focal length, can be utilized advantageously, in particular, in operating microscopes. The focal length of the principal objective in fact influences a number of parameters and primary optical quantities of the objective or the microscope, respectively, including among others the working distance. The working distance represents the shortest distance between the object plane and the objective of a microscope. The working distance thus determines the free working space between object field and operating microscope, which is available to the surgeon, as well as the distance between the surgeon's eyepiece view and the object field.
In order to be able to make available the necessary working distance for the surgeon for different situations, exchangeable objectives have therefore been developed for varying the working distance, by means of which the principal objective of one focal length can be replaced by a principal objective of another focal length. Depending on the respective professional discipline, exchangeable objectives with focal lengths of, for example, between approximately f=150 mm and 500 mm are used as exchangeable objectives.
An exchangeable objective is in general a simple cemented member and thus the working distance is approximately equal to the focal intercept of the objective, thus equal to the distance from the vertex of the lens on the object side to the focal point in the object plane and approximately equal in magnitude to the focal length of the objective.
In addition to the working distance, thus the focal intercept of the objective, there are also other primary optical quantities of the operating microscope, however, that are of importance to the surgeon. These include, for example, the total magnification, the stereo angle, the diameter of the object field, the resolution of the object, the depth of sharpness and the like. These primary optical quantities, of course, also depend on the focal length of the objective used. If an objective of another focal length is used for variation of the working distance, then these optical quantities are also influenced due to the change in focal length.
For example, the total magnification is inversely proportional to the focal length of the objective. Also, the stereo angle determining the stereo impression is inversely proportional to the focal length of the objective.
In order to be able to avoid changing the objective, as must be performed in the case of an exchangeable objective, objectives have been developed in which variable working distances can be provided with a single objective. This represents a great practical advantage in several fields of application of microsurgery, whereby still even today, after more than a decade from the first application, this type of objective continually finds new fields of application. These objectives usually have a two-part construction and comprise an optical member with negative refractive power and an optical member with positive refractive power. Depending on the arrangement of these two members, the objectives are designated as objectives according to the telescopic system or objectives according to the retrofocus system.
Objectives with variable working distance are also designated as varioscopes in the following. A varioscope is described, for example, in DE 2,439,820 A1. This objective is comprised of a movable positive member and a stationary negative member, which are separated from one another by a distance. The optical construction is in principle that of a telescopic system, i.e., the negative lens is introduced on the object side. The positive member is comprised of a cemented member and a single lens, has a focal length of 110 to 130 mm and can be moved about 16 to 20 mm for variation of the working distance. The stationary negative member is comprised of a cemented member and can be exchanged in order to expand the working distance. With a focal length of 120 mm for the positive member and a displacement path of 16 mm also for the positive member, the working distance can be shifted from 160 mm to 220 mm, with a negative member with the focal length of −160 mm. Under otherwise equal conditions, the working distance can be shifted from 197 mm to 300 mm with a negative member with the focal length of −200 mm.
An objective of the varioscope type according to the telescopic system is also described in the German Utility Model G 90 16 892.5. Here a negative lens member with a focal length between −105 mm and −135 mm is arranged on the front side, i.e., turned toward the object plane, and a positive lens member comprised of a cemented member and a single lens with a focal length between 100 mm and 120 mm is arranged so that it can be moved thereunder. The displacement path of the positive member lies between 21 mm and 30 mm. Without an exchange of optical members, this varioscope has a working distance range of 150 mm to 450 mm.
In comparison to principal objectives with a fixed focal length, which are generally comprised only of a single cemented member, in the case of varioscopes, the primary optical quantities are considerably varied. These differences are a logical consequence of the discrepancy between the focal length and the focal intercept, i.e., approximately the working distance which is present in a varioscope.
While in the case of an exchangeable objective with a fixed focal length, the focal length is approximately equal to the focal intercept and thus equal to the working distance, in the case of telescopic systems of a varioscope, the focal length is clearly greater than the working distance. Therefore, the total magnification, the stereo angle, the resolution of the object, the depth of sharpness and also the diameter of the object field are also changed when compared to a principal objective with a fixed focal length.
With a longer focal length, in particular, a smaller total magnification and a smaller stereo impression are obtained. For this reason, which is described in U.S. Pat. No. 5,701,196, the problem which is the basis of the invention is to create a focal length, which is shorter than the working distance, with an objective lens system. Additionally, it should be possible to change the working distance. This problem is solved according to the publication by employing either an objective lens system which possesses a modified telescopic system construction, or an objective lens system which has the construction of a so-called retrofocus system, i.e., an inverted telescopic system. In the modified telescopic system described in U.S. Pat. No. 5,701,196, a very concavely curved surface, by means of which the principal plane of the objective is displaced, and thus the focal length of the objective can be shortened, is introduced on the first surface of the positive member. In the case of the retrofocus system, the effect is obtained by introducing the positive member on the object side.
Also in the case of the retrofocus system, there exists, of course, a discrepancy between the focal length and the focal intercept, thus also the working distance, whereby in the case of the retrofocus system, the focal length is clearly shorter than the working distance; the principal plane thus lies between the object plane and the objective.
Such varioscopes according to the retrofocus system introduce several advantages. In particular, as described in U.S. Pat. No. 5,701,196, an increase in the total magnification and the stereo impression are obtained with such varioscopes. Of course, these systems have the disadvantage that they possess a smaller object field diameter and a smaller depth of sharpness than telescopic systems. In several cases this can be undesired by the user of a microscope with such an objective, for example, by a surgeon, or it may be perceived as troublesome.